This within‐family study investigated whether mothers differentiate between children in their interactive behavior. Mothers were observed during a play and a task interaction separately with their child with autism spectrum disorder (ASD) (between 46 and 84 months old, M = 68) and with a younger sibling (between 29 and 67 months old, M = 48). Additionally, the social behavior of the children with ASD and their non‐ASD siblings was compared. Results show that mothers differentiated in their responsiveness but not in their initiatives toward the children. Children with ASD and their non‐ASD siblings were equally responsive but children with ASD were more imperative toward their caregiver. Several interpretations of these findings are discussed. Finally, it is concluded that family‐based interventions will benefit from a better understanding of the effect of ASD on mother–child interactions within families with a child with ASD. Therefore, between‐family studies should be complemented with within‐family studies. Autism Res 2011,4:358–367. © 2011 International Society for Autism Research, Wiley Periodicals, Inc.